An expression used to tell someone to be careful.
Also can be in the form of 'Don't CATCH AIDS.

Guy at party: Gonna be diggin that girl out soon.

His Friend: Cool, use rubber and don't get AIDS.

Ty: Going rock climbing on Saturday

His mom: K son, just don't get AIDs. *mwah* Love you.

Ty: WTF!!!?

WWII Marine Captain: Now remember men, we're completely surrounded by the enemy. Use you head or the snipers will make your you lose it. Just don't catch AIDs and we'll be home to see our families by Christmas. Clear?